X-ray crystal spectroscopy has played an important role in the diagnostics of tokamak fusion plasma, in particular, by Doppler measurements of the central ion temperature, which was, in general, determined from the Doppler width of the resonance lines of the helium-like ions of medium-Z elements, like Ar, Ti, Cr, Fe and Ni. The observed satellite spectra of these heliumlike ions also provided information on various other plasma parameters, e.g., central electron temperature, ionization balance and ion impurity transport.
X-ray crystal spectroscopy is expected to become even more important for the diagnostics of future large hot plasmas, such as will be produced in the International Thermonuclear Experimental Reactor (ITER), because other diagnostic methods will encounter technical difficulties and not be able to measure the central plasma parameters of this fusion device. X-ray lines from high-Z elements such as krypton in the He-like charge state will be well-suited for measurement of the central ion temperature and other central plasma parameters at ITER.
However, the requirements for X-ray crystal spectrometers will change as experimental fusion plasmas increase in size and approach break-even conditions. Under these conditions, the plasma will emit neutrons and gamma radiation. It will therefore be necessary to restrict the size of diagnostic windows and to place diagnostic instruments at large distances from the plasma. Moreover, the concentration of high-Z impurities in the hot core of the plasma will have to be as small as possible, in order to avoid an unacceptable dilution of the burning hydrogen plasma. This latter requirement, especially the intensity of the X-ray line radiation which is needed for plasma diagnostics, will be severely reduced. For the X-ray diagnostics of the core plasmas in future large fusion devices, such as ITER, it is therefore necessary to develop new types of crystal spectrometers having improved focusing properties and which employ very large crystals to enhance the spectrometer throughput or the X-ray flux to the detector.
Crystal spectrometers with improved focusing properties are also needed for the diagnostics of present-day tokamak and stellarator plasmas to satisfy the demands for advanced diagnostics having good spatial resolution and which are capable of providing an image of an extended plasma. The requirements that apply to an imaging X-ray crystal spectrometer for the present-day plasma devices are, however, distinctly different from the requirements for future large devices, such as ITER, which are discussed above. The experimental constraints at present devices are less restrictive because the background of neutron and gamma radiation is orders of magnitude smaller than expected in ITER. The diagnostic windows are larger and the diagnostic instruments are closer to the plasma than they will be in ITER. It is therefore possible to view a large part of present plasmas and to obtain information on the spatial distribution of plasma parameters.
The presently used instruments are Johann spectrometers with cylindrically bent crystals. These instruments only provide focusing for the meridional rays, which are parallel to the main diffraction plane, whereas the sagittal rays, which are oblique to the plasma, are not focused. With exception of the experimental arrangement by Bartiromo et al. (see discussion below), information on the spatial distribution of plasma parameters was previously obtained by arrays of such Johann spectrometers, where each spectrometers records spectra from a single line of sight through the plasma. The number of spectrometers and thus the number of lines of sight, which can be installed at a tokamak is limited and usually not larger than five, because of experimental and budget constraints. The experimental arrangement by Bartiromo et al. also uses a Johann spectrometer with a cylindrically bent crystal. However, in Bartiromo's arrangement, there is a horizontal slit between the crystal and the plasma, so that rays, which emanate from different parts of the plasma above or below the main diffraction plain pass through the slit and are reflected from different parts of the crystal below or above the main diffraction plain onto the detector. Although this arrangement provides spatial resolution in the plasma, the throughput of the instrument is significantly reduced by the slit.
The present invention addresses the limitations of the prior art in terms of future more demanding applications by providing a doubly focusing crystal spectrometer capable of providing simultaneous measurements of spectra from an essentially unlimited number of lines of sight (or chords) through the plasma and thus produce an image of the plasma in a direction perpendicular to the main diffraction plane.